Echelle spectroscopy and BVRI photometry of the rapidly evolving PNV J23244760+6111140

ATel #14476; U. Munari (INAF Padova), P. Valisa and S. Dallaporta (ANS Collaboration)
on 19 Mar 2021; 22:17 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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The transient PNV J23244760+6111140 has been discovered at unfiltered 9.6mag on 2021 March 18.424 by Yuji Nakamura (Japan) on frames taken with a 135mm/F4.0 lens with a CCD. A nova classification for the transient has been proposed in ATel #14471, #14472 based on the presence of emission lines of Balmer series, 4640 blend, He II 4686, and He I, the latter with P-Cyg profiles.

We have obtained BVRI photometry of the transient with the ANS Collaboration telescope ID 0310, with color equations on the Landolt system calibrated against field stars extracted from APASS DR8 sky survey. The results for March 19.762 UT are B=8.223, V=7.717, R=7.240, and I=6.935 (errors <0.01 in all bands). The transient appears to have significantly brightened from the B=9.58, V=9.27, and R=8.78 reported to CBAT by N. Ikonnikova and K. Sokolovsky for March 18.684 UT.

A high resolution Echelle spectrum (range 4300-8700 Ang, res. power 20000) has been obtained on March 19.785 UT with the Varese 0.84m telescope. The spectrum shows a large evolution since that described in ATel #14471, which was obtained a day earlier on March 18.768. The high ionization emission features, namely the HeII 4686 line and the blend at 4640 Ang, have gone, and the spectrum is now dominated by hydrogen Balmer lines and HeI lines of great intensity, especially 5876, 4471 and 5016 Ang. P-Cyg absorption components are present to several lines, but not to all. Such P-Cyg present a lot of sub-structures, with the main component at -1525, -1500, and -1510 km/s from the emission peak for Halpha, Hbeta and HeI 5876, respectively. Hydrogen (both Balmer and Paschen series) and HeI lines make essentially all lines that are visible. Their profile is best described by a Voigt function, with rather extended wings (3000 km/s pedestal width) compared to a relatively narrow core.

Strong interstellar lines from NaI doublet and KI 7698 Ang are visible, with at least two components, at heliocentric velocity -53.5 and -14.0 km/s. Their equivalent widths for the D1 line are 0.264 and 0.635 Ang, respectively, with the latter appearing obviously saturated. Adopting the Munari and Zwitter (1997A&A...318..269M) calibration, the reddening from the unsaturated NaI D1 component at -53.5 km/s is E(B-V)=0.08, and E(B-V)=0.47 from the unsaturated KI component at -14.0 km/s, for a total reddening of E(B-V)=0.55 affecting PNV J23244760+6111140. Such an amount is supported by the presence of many diffuse interstellar bands (DIBs) visible in our spectrum, primarily 5780 and 5796 Ang, of equivalent width 0.334 and 0.128 Ang respectively. The DIB at 6614 Ang is seen with an equivalent width of 0.116 Ang; adopting the calibration by Munari (2014ASPC..490..183M), it translates to a reddening of E(B-V)=0.51, in nice agreement with the combined result from NaI and KI interstellar lines.

From our photometry and E(B-V) estimate, the reddening-free colors of PNV J23244760+6111140 are B-V=-0.04, V-R=+0.13, and V-I=0.00.